1. Field of The Invention
The present invention relates generally to an optical deflector for deflecting light, such as laser beams, by slightly displacing a light deflecting member, and a beam scanner using the optical deflector.
2. Related Background Art
In recent years, with the development of various apparatuses using laser beams or the like, various uses of optical deflectors have been developed. For example, optical deflectors are used for laser beam scanners of conventional laser printers which are output units capable of outputting any numbers of pages of picture and/or document information every page.
As a typical optical deflector, the construction of a conventional galvanomirror for use in a maltibeam scanner will be described below. FIG. 12 is a perspective view of a galvanomirror, FIGS. 13A through 13C are views showing three faces thereof, and FIG. 14 is an exploded perspective view thereof.
In FIGS. 12 through 14, a mirror (a light deflecting member) 20 for deflecting a laser beam is elastically supported on an elastic supporting member, which itself is slightly moved. This elastic supporting member is a plate spring (a supporting member) 21 which comprises: a portion mounted on the mirror 20; a portion mounted on a yoke 24 serving as a frame; and two torsion springs, i.e., torsion bar springs 22a and 22b, for connecting the portion mounted on the mirror 20 to the portion mounted on the yoke 24. The mirror 20 is bonded to the plate spring 21. The reflecting surface, i.e., the evaporation surface, of the mirror 20 is provided on one side of the mirror 20, the other side of which faces the plate spring 21.
The plate spring 21 is made of a beryllium copper or stainless, e.g., SUS304, which are often used as materials of a spring. A bobbin 23 is bonded to one surface of the plate spring 21, to the other surface of which the mirror 20 is bonded. A coil 26 is bonded to the inner surface of the bobbin 23. The plate spring 21 is fixed to the yoke (a holding member) 24 of a ferromagnetic material also serving as a frame, by means of plate spring presser members 25a and 25b of a resin. That is, the plate spring 21 is fixed to the yoke 24 by the engagement of screws (not shown) with threaded holes 40a and 40b formed in the yoke 24, holes 37a and 37b formed in the plate spring 21, and holes 41a and 41b, which are formed in the plate spring presser members 25a and 25b, respectively.
A magnet 27 is bonded to a magnet fixing plate (a base) 28 of a non-magnetic material, and fixed to the yoke 24 by the engagement of screws (not shown) with threaded holes (not shown) formed in the yoke 24 and holes 39a and 39b formed in the magnet fixing plate 28. The torsion bar springs 22a and 22b are provided on both sides of the plate spring 21 in longitudinal directions thereof, so that the mirror 20 is rotatable in the directions of arrows R (see FIGS. 12 and 13C).
When an electric current flows the coil 26, an electromagnetic force is produced between the coil 26 and the magnet 27 mounted on the magnet fixing plate 28 to rotate the mirror 20 in the directions of arrows R.
Specifically, referring to FIG. 15, the lines of magnetic force leave the N pole surface (a polarized surface) of the magnet 27 for the yoke 24, and then, the lines of magnetic force are divided by the yoke 24 into right and left parts as shown by arrows B to go half around the magnet 27 along the yoke 24 to reach the opposite S pole surface (a polarized surface) of the magnet 27.
When an electric current flows the coil 26 in this state, magnetic forces are applied to the linear portion of the coil 26 arranged at the magnetic gap between the magnet 27 and the yoke 24 on the side of the N pole surface and to the linear portion of the coil 26 on the side of the S pole surface, and the directions of the magnetic forces are opposite to each other.
Therefore, since the plate spring 21 is fixed to the yoke 24, the plate spring 21 rotates around the torsion bar springs 22a and 22b serving as torsion springs, and the mirror 20 also rotates in the directions of arrows R. Furthermore, if the direction of the electric current is changed, the direction of rotation can be changed, and the rotation angle can be changed in proportion to the current value. In addition, the rotation angle of the mirror 20 can be maintained by holding the passing current.
Referring to FIGS. 13A through 13C, the gaps between the yoke 24 and the bobbin 23 are filled with damping materials 29a and 29b of, e.g., silicon gel, so as to prevent the mirror 20 from being vibrated by disturbance vibration.
However, when a laser beam is scanned by the optical deflector with the above construction, there are the following problems.
That is, referring to FIG. 14, positioning pins (positioning members) 31a and 31b are inserted into positioning holes 38a and 38b of the magnet fixing plate 28 and positioning holes 33a and 33b of the yoke 24 in order to position the yoke 24 with respect to the magnet fixing plate 28. However, since the conventional positioning pins 31a and 31b are made of SUS304 or the like which is a non-magnetic material, there is a problem in that the lines of magnetic force passing through the yoke 24 are obstructed, so that the efficiency of a magnetic circuit formed in the yoke 24 is deteriorated.
In addition, referring to FIG. 15, the lines of magnetic force leave the N pole surface of the magnet 27 for the yoke 24, and then, the lines of magnetic force are divided by the yoke 24 into right and left parts as shown by arrows B to go half around the magnet 27 along the yoke 24 to reach the opposite S pole surface of the magnet 27. However, when the lines of magnetic force leave the N pole surface for the yoke 24, the lines of magnetic force go while being expanded since the area of a portion of the yoke 24 facing the N pole surface of the magnet 27 is greater than the area of the N pole surface. The side of the S pole is the same. Therefore, there is a problem in that the magnetic flux density at the coil 26 decreases so as to deteriorate the efficiency of the magnetic circuit.
In addition, referring FIG. 15, the lines of magnetic force leave the N pole surface of the magnet 27 for the yoke 24, and then, the lines of magnetic force are divided by the yoke 24 into right and left parts as shown by arrows B to go half around the magnet 27 along the yoke 24 to reach the opposite S pole surface of the magnet 27. However, there is some possibility that some lines of magnetic force returning to the magnet 27 without going half around the magnet 27 may occur at projections 47a and 47b which are provided in the yoke 24 for filling silicon gel so as to prevent the vibration. Therefore, there is a problem in that leakage flux may occur to deteriorate the efficiency of the magnetic circuit formed in the yoke 24.